Interface microstructures in the diffusion bonding of a titanium alloy Ti 6242 to an INCONEL 625
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I.
INTRODUCTION
S U R V E Y S carried out by some oil companies have shown a recent and clear trend toward drilling wells to greater depths. As the drilling for oil and gas gets deeper, the requirements of materials for tubing become more stringent, due to the rise in temperature and pressure and also because the tubes have to sustain their own weight. An additional problem with deep oil and gas wells is severe corrosive attack due to H2S, HC1, and CO2 in this high-pressure and high-temperature environment. Although chemical inhibitors have been used for the purpose of alleviating this problem, they have been shown to be both expensive and ineffective. Stainless steels, c~1 superalloys, t~'21 and zirconium t31 have been utilized, but achieving the combination of strength and corrosion resistance in a single material is difficult. Overcoming such difficulties has encouraged the development of bimetallic materials: a steel of high strength metallurgically bonded to a thin corrosion-resistant-clad alloy, produced in an economical way (e.g., extrusion). The potential of these products is very large and they could be also used in other applications, such as pipelines for oil transport, heat exchangers, and in the chemical industry and cryogenics field, among others. Given that titanium alloys present a good combination of strength and creep resistance, together with excellent resistance to corrosion, they are good candidates to be used as corrosion-resistant-clad alloys. Several combinations of steels and titanium alloys have been analyzed,f4 9] and in a recent article, the combination of Ti6242 with austenitic stainless steel has been fully analyzed, t~~ Several undesirable brittle phases, such as a sigma phase, have been identified inside the bonding. In the present work, diffusion bonding between an B. ALEMAN, Ph.D. Student, I. GUTI[~RREZ, Senior Researcher, and J.J. URCOLA, Head of the Department of Materials, are with Centro de Estudios e Investigaciones Trcnicos de Guipuzcoa, Apartado 1555-20009 San Sebastian, Basque Country, Spain. Manuscript submitted May 12, 1993.
METALLURGICAL AND MATERIALS TRANSACTIONS A
INCONEL* 625 and a Ti6242 titanium alloy has been *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
studied. The complicated combination of microstructures (the combination of a high number of intermetallics) developed during the diffusion bonding by uniaxial compression of Ti6242 alloy to an INCONEL 625 is described in detail. II.
EXPERIMENTAL PROCEDURE
The materials used in the present work were a Ti6242 alloy and an INCONEL 625 superalloy (their chemical compositions are given in Table I). They were supplied in the form of 16-mm-diameter commercial rods. Pieces 25-mm high for INCONEL 625 and 4.2-mm high for Ti6242 were cut from the rod and mech.anically polished on SiC emery paper, followed by diamond paste, using a cloth. The polished pieces were ultrasonically cleaned in an acetone bath. Three pieces (in the following order: INCONEL, Ti alloy, INCONEL) were introduced int
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